The control of electrical loads to ensure they are only active when required is a known problem. Solutions to this problem are advantageous in minimising the cost and environmental effect of powering such loads. For example, in lighting control systems there is a desire to only turn on the lights when the space to be illuminated is occupied. A common solution involves a passive infra-red (PIR) sensor used to detect the movement of a person within range of the sensor. A living person is warm and therefore emits infra-red (IR) radiation. The PIR sensor typically features a lens which focuses the incident IR radiation onto one of a plurality of sensitive elements behind the lens. Adjacent sections of the lens focus the IR radiation to different IR-receptive elements. By detecting relative changes in the output signals from the sensitive elements, PIR sensors can determine movement of a person within the detection field of the sensor. However, a PIR sensor of this type is unable to detect a person that is not moving.
An alternative solution uses a microwave sensor which emits radiation at microwave frequencies and detects reflected radiation from objects within the detection range of the sensor. For moving objects the radiation is reflected at a different frequency to the incident radiation due to the Doppler effect. By comparing the frequency of the emitted radiation to that of the reflected radiation, movement within the detection range of the microwave sensor can be detected. Microwave sensors typically have longer ranges than PIR sensors. Without expensive modification, microwave sensors are not suitable for detecting the presence of a person who is not moving.
Ultrasounds sensors can also be used to detect the presence of a moving person. In a similar way to microwave sensors, the ultrasound sensor transmits ultrasound, and the frequency of the received echo can be used to determine the presence of a moving target. As with the PIR and microwave sensors, ultrasound sensors are also not generally suitable for detecting the presence of a person who is not moving.
Recently, thermopile array sensors have been developed. A thermopile array is made up of several thermocouples. A thermocouple uses the fact that a temperature difference between two dissimilar conductors in contact will produce a potential difference across the junction. A thermopile uses this principle to generate a potential difference in response to incident infra-red (IR) radiation on a contact plate, representing the hot side of the junction. The other conductor, which is in contact with the hot side, is shielded from the incident radiation and so does not heat up. By placing the hot side in a vacuum behind an IR-transparent lens, the device can be made more sensitive, since the heat does not dissipate through convection. IR radiation from different areas can be directed towards different thermopiles in the array. In this way, a ‘pixel’ corresponding to the area from which the IR radiation originated can be determined. Unfortunately, thermopile arrays typically have a detection range shorter than current sensors of other types. In addition, their response times are relatively long, which makes them difficult to use in many lighting applications.